Liquid crystal image display systems have been increasingly used for display of graphic, symbolic, and TV pictorial images. Among the advantages of such display devices are high brightness, large scale display capabilities, compact size, and high resolution.
As is well known, the liquid crystal light valve is a thin film, multilayer structure comprising a liquid crystal layer, a dielectric mirror, a light blocking layer, and a photoresponsive layer sandwiched between two transparent electrodes. A polarized projection beam is directed through the liquid crystal layer and to the dielectric mirror. An input image of low intensity light, such as that generated by a cathode ray tube, is applied to the photoresponsive layer thereby switching the electric field across the electrodes from the photoresponsive layer onto the liquid crystal layer to activate the liquid crystal.
Linearly polarized projection light passing through the liquid crystal layer and reflecting from the dielectric mirror is polarization-modulated in accordance with the information incident on the photoconductor. Upon exiting the light valve, the light is converted by means of a crossed polarizer to an intensity-modulated beam which is projected by a lens onto a screen. In practice, the polarizing and analyzing functions are accomplished by a single polarizing beamsplitter. The intensity of the projected image has a point-to-point correspondence to the intensity of the writing light input image. Therefore, if a complex distribution of light, for example, a high resolution input image, is focused onto the photoconductor surface, the device converts that image into a high brightness replica which can be projected with high magnification to produce a high brightness image on a viewing screen.
This operation is further described in U.S. Pat. No. 4,019,807 issued to D. Boswell et al on Apr. 26, 1977, and assigned to the assignee of the present invention.
A graphics display projector using a liquid crystal light valve of the above type is described in an article entitled "Application of the Liquid Crystal Light Valve to a Large Screen Graphics Display", published in the 1979 Society for Information Display (SID), International Symposium, Digest of Technical Papers, May 1979, pp. 22-23. This display system, a type with which the present invention is particularly but not exclusively concerned, projects a large scale image having yellow-alpha numeric characters on a blue background. The colors are a result of the unavoidable fact that the liquid crystal material polarization modulates the white projection light incident upon it as a function of the wavelength of the light. Although providing an image of high brightness and high resolution, the system has several inherent drawbacks which it is the object of the present invention to minimize if not eliminate. One drawback stems from the fact that the light valve is a complicated, expensive device that includes numerous microscopically thin film layers deposited on super-flat substrates, each requiring a series of critical manufacturing steps. Among these steps, the two substrates which sandwich the few micrometerthick liquid crystal layer must be polished over their approximately five centimeter faces to an optical flatness of better than approximately one-quarter the wavelength of white light or 0.15 micrometers. Additionally, the device must be assembled so that these two optically flat substrates, to define the liquid crystal layer thickness, are separated by precisely several micrometers and uniform from point to point to within a fraction of a micrometer. As will be further described, the nematic (rod-like) liquid crystal molecules comprising this layer are arranged in chains which are precisely oriented in twist and tilt angles to the substrates.
The difficulty of producing such a complicated sutructure within the required tolerances results in a large percentage of defective liquid crystal valves, thus raising their effective unit cost. In effect, variations in the liquid crystal layer thickness, caused by both surface waviness and wedging of the substrate surfaces which contain the liquid crystal material, create poor uniformity of the image background color. Non-uniform twist and tilt of the liquid crystal molecules in their off-state also gives rise to color nonuniformities in the image background. An additional cause of such uneven color is residual birefringence within the polarizing beam splitter. This residual birefringence can occur from manufacturing imperfections as well as from nonuniform heating by the heat sources within the image projector package, such as electronics and the high intensity projection light source. In particular, as a consequence of the variations in liquid crystal tilt and twist variations in the liquid crystal layer thickness, the image background will appear not in the desired uniform shade of blue, but rather with spatial variations in brightness and in color. These color variations typically ranging from violet to magenta and green, create a very distracting visual effect, thereby interfering with effective communication.
In addition to the above described limitations caused by manufacturing tolerances, another limitation is the strong dependence of display background color on liquid crystal layer thickness. Thus, the desirable color contrast of yellow characters on a blue background is achieved only by using a relatively thick liquid crystal layer of from approximately six to eight micrometers. Because the time response of the liquid crystal layer varies as the square of its thickness, the blue color is achieved at the expense of unsmeared video rate images. Although video rate images can be produced with a thinner liquid crystal layer of from approximately three to four micrometers, the disadvantage is that the background color, being controlled by the thickness of the liquid crystal, appears black with yellow characters. This color combination not only gives a relatively low color contrast, but is visually unpleasing.
A still further limitation is that even though a given device may have a uniform blue background, the blue can vary in shade from device to device.